Phased array radar system for tracking

ABSTRACT

A phased array radar system for target tracking having a track initiation unit, a track prediction unit, a scheduling unit, a track selection unit, and a transmitter/receiver unit. The track initiation unit initiates new tracks representing detected aircraft targets. The track prediction unit predicts the expected position and the calculated position uncertainty of the target as a function of time and the minimal, maximal and optimal time difference to the next measurement. The scheduling unit performs an independent calculation of a sequence of possible time intervals to the next measurement in accordance with specified conditions, and then performs an intersection operation between the calculated sequences of time intervals in order to calculate the optimal time interval to the next measurement. A track selection unit selects that track which has the shortest remaining time interval, K i , to the next measurement and decreases the time interval to the next measurement for all other tracks with K i . The transmitter/receiver unit registers an echo from the target and calculates, in a known manner, values for distance, speed, bearing and uncertainty in the distance and speed calculations, which are transferred to the track prediction unit for further calculations.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a phased array radar tracking system.For radar technical terms in the following description a basic textbookis recommended, e.g. S.Kingsley and S.Quegan, Understanding RadarSystems, McGraw Hill, 1992.

2. Description of the Related Art

The proposed radar design is intended for tracking aircraft targets. Theradar system has a phased array antenna, which means that it can becontrolled and directed electronically. Each detected aircraft target isfollowed and represented as a track. The track is a state vector withelements for a set of parameters. The main way of working for the radaris to transmit pulses with a certain pulse repetition frequency andcarrier frequency, in a certain direction. After being reflected againsta radar target (subsequently called "the target") they may be measuredby a receiver. The time delay from transmission to reception of a pulseis proportional to the target distance.

The distance is, however, ambiguous since the pulse frequency value isso high that several pulses are transmitted before the reflection of thefirst pulse returns. This ambiguity gives rise to an ambiguity problemin calculation of the distance: Each measured time between transmissionand reception of a pulse corresponds to several possible ranges. Thetwo-way distance that a certain radar pulse can go in the time intervalbetween two consecutive pulses is called the range-unambiguity interval.The length of the range-unambiguity interval depends on the value of thepulse repetition frequency (PRF). For a radar of this type the number ofselectable PRF values usually amounts to some tens. A sequence of pulsestransmitted with a certain PRF is called a pulse train. Between eachpair of adjacent range-unambiguity intervals there is a blind regiondependent on the fact that it takes a certain time to transmit the pulsefrom the antenna.

Before each new measurement of a target the position of the target aswell as the position uncertainty is predicted. A common computationtechnique for this prediction is Kalman filtering. The positionuncertainty forms an uncertainty volume (or uncertainty-region), whichgrows roughly quadratically with the time since the latest measurement.In order to master both the uncertainty about the target position andthe radar-target range-unambiguity, it is necessary that the extensionof the uncertainty volume--along the radius between target and radar--iscontained completely in one single range-unambiguity interval. Due tothe (predicted) target movement in relation to a possibly moving radarthis condition is satisfied only during some limited time intervals,namely such time intervals for which the radar-target range and theposition uncertainty region for the target lies completely within thelimits of one single range-unambiguity interval, depending on theselected PRF value.

How this PRF value, these time intervals and the time point formeasurement shall be calculated is one of the problems that have to besolved in a scheduling device of a phased array radar. Several otherfactors must however also be considered in this calculation.

One factor is associated with a combination of Doppler frequency shiftsand ground echo cancellation. If a movement of a target between twopulses in the pulse train equals in radial direction a number of halfwave lengths for the carrier wave, the target seems to stand still. Foreach PRF a number of (equally large) speed unambiguity intervals arises,during which the target speed is unambiguous. Furthermore, all echoesfrom slow targets must be cancelled (ground echo cancellation). Thecombination of these two effects leads to "blind spots" in the speedspectrum. This phenomenon is called Doppler blindness.

A third problem occurs as the radar can measure only one target at eachinstance. For each track to be scheduled, the measurement time intervalsassigned to other tracks are already occupied.

A fourth problem is caused by so called "cross-overs": If the positionuncertainties of more than one target during some time interval lie inbeam sectors that overlap this time interval will be impossible to usefor measurement.

The common thing with all these problems is that they depend on thechoice of PRF value and scheduled measurement time interval. The problemcomplexity may grow as further conditions may have to be added on theradar. The problem complexity is a new one as phased array radars arenew. Known approaches to solve the problem handle one track at a time byfirst assigning to it the next free time period, and thencalculating--if possible--the PRF value that fulfils all demands. Inanother approach it is determined which tracks need to be measured eachtime the radar is "free", which of these tracks that it is mostnecessary to measure and then a PRF value for this track is calculated.

SUMMARY OF THE INVENTION

The proposed scheduling mechanism solves the stated problem complexityin an optimal and simple way which in addition allows more constraintsto be added. This is achieved by giving the invention the design of aphased array radar system for target tracking having a track initiationunit, a track prediction unit, a scheduling unit, a track selectionunit, and a transmitter/receiver unit. The transmitter/receiver unitforms a waveform and directs a beam toward a target. The trackinitiation unit initiates new tracks representing detected aircrafttargets. The track initiation unit is connected to a track predictionunit which predicts the expected position and the calculated positionuncertainty of the target as a function of time and the minimal, maximaland optimal time difference to the next measurement. A scheduling unit,which is connected to the track prediction unit, performs an independentcalculation of a sequence of possible time intervals to the nextmeasurement for each one of two conditions, namely that the measurementtime difference will be placed between the calculated minimal andmaximal time differences and that range-unambiguity will prevail, andthen performs an intersection operation between the so calculatedsequences of time intervals in order to calculate the optimal timeinterval to the next measurement. A track selection unit, connected tothe scheduling unit and to the transmitter/receiver unit, selects thattrack which has the shortest remaining time interval, K_(i), to the nextmeasurement and decreases the time interval to the next measurement forall other tracks with K_(i). The transmitter/receiver unit, afterdirecting the waveform beam to the target, registers an echo from thetarget and calculates, in a known manner, values for distance, speed,bearing and uncertainty in the distance and speed calculations. Thesevalues are transferred to the track prediction unit for furthercalculations.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described with references to theattached figures, where

FIG. 1 shows the principal structure of one embodiment of the invention,

FIG. 2 shows the principal structure of one embodiment of the schedulingunit 2 in FIG. 1,

FIG. 3 shows the geometrical relations for an approaching target, wheret_(i) ^(k) denotes the start of time interval number k, if i=1, and theend of the same, if i=2,

FIG. 4 shows how the beam for target i coincides with the beam fortarget and

FIG. 5 shows a picture of the range-unambiguity interval at threedifferent PRF values, where bold lines denote blind parts of therange-unambiguity in each interval, (a) denotes a position which isblind free at only one of the three PRF values, and (b) and (c) denotepositions which are blind free in two and three of the investigated PRFvalues respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention consists of a device which computes the time points andPRF values for updates of a number of established target tracks, bycalculation of a time interval sequence for each one of a number ofgeometrical and other conditions which may be imposed on the measuredtarget in relation to the position and speed of the radar itself. Thesetime interval sequences are then intersected forming a new time intervalsequence within which the optimal measurement time interval can besought.

On a basic level the radar system according to the invention comprises atrack initiation unit 6, a track prediction unit 1, a scheduling unit 2,a track selection unit 3 and a transmission/receiving unit 4, as shownin FIG. 1.

The track initiation unit 6 initiates new tracks. It is connected to thetarget prediction unit 1 which predicts the expected position of thetarget and the calculated position uncertainty at the next measurementas a function of time and furthermore the minimal, maximal and optimalmeasurement time difference.

The track prediction unit 1 is connected to a scheduling unit 2, whichmakes an independent calculation of a sequence of possible timeintervals to the next measurement for two conditions, namely that themeasurement time difference has a value between the minimal timedifference and the maximal time difference, and that range-unambiguityshall prevail. The unit then makes an intersection operation on the twoso calculated interval sequences in order to calculate the optimal timeinterval to next measurement.

The scheduling unit 2 is connected to a track selection unit 3 whichselects the track which has the shortest remaining time interval, K_(i),to the next measurement and decreases the time interval to nextmeasurement for all other tracks with K_(j). The track selection unit 3is connected to a transmission/receiving unit 4 which generateswaveform, directs the beam, is designed to register the reflex from thetarget and calculate, in a known way, range, speed, angle and accuracyin the calculations of range and speed, which parameters are transferredto the track prediction unit 1 for use by the track prediction unit forconducting further calculations.

In certain cases the scheduling unit 2 is not able to calculate any timeto the next measurement for any PRF value. In order to manage thesecases it is suitable for the radar system to comprise a resolution unit5 which is connected to the scheduling unit and which can calculateresolution frequencies and possible measuring times for resolution.

Data necessary for the calculations in the different units and for theresults from the different units must be stored in some way. It issuitable to use a special memory unit 7 which is connected to all otherunits for this purpose. Other solutions for providing memory managementare however also possible.

In the following paragraphs the different units will be presented inmore detail.

The track initiation unit 6 initiates tracks. This is done by using anytrack detection method which gives initial values of the parametersneeded to perform track prediction in track prediction unit 1.

The track prediction unit 1 calculates or selects a number of parameterswhich later will be used by the transmission/receiving unit 4. Using thevalues obtained from the latest measurements, the target is assigned atarget track hypothesis. This can be a straight flight hypothesis or amaneuver hypothesis. The latter might be of different types depending onthe turning forces that influences the target. A straight flighthypothesis is characterized by a slow rate of growth in positionaluncertainty, and a maneuver hypothesis is characterized by a faster rateof growth in uncertainty--the bigger the hypothetical turning force, thefaster the rate of growth in uncertainty. This growth of uncertainty isexpressed by the factors μ_(l) (t) and μ_(u) (t) in the formulas a₁ anda₂ below, and is illustrated by the breadth of the trumpet shapedopening in FIG. 3. The faster the rate of uncertainty growth, the largerthe trumpet opening. For assignment of hypothesis, several knownstatistical methods are known, the most of which are based on theresults of the latest measurements and their position and distance inrelation to hypothetical straight flight tracks or turning curves.

Based on the target track hypothesis it is possible to determine some ofthe factors needed later in order to estimate the expected position ofthe target at next measurement--for example in terms of polarco-ordinates. Some of these factors are position, speed, turning rateand acceleration of the target at the latest measurement and functionsfor estimating the corresponding values at the next measurement. Oneparameter of these functions is the so far unknown value of the timedifference between the latest and the next measurement for the currenttrack. Values on position, turning rate and acceleration of the trackare written in the corresponding memory cells for the actual track inmemory unit 7. For straight flight hypothesis the function for positiondecision is indicated in the right part of formula a₁.

The positional uncertainty depends, among other things, on the timedifference between the latest and the next measurement for the currenttrack. The growth in the positional uncertainty can be achieved indifferent ways, for example with the right part of the formulas a₁ anda₂ below, or with Kalman filtering. Necessary parameters--with theexception of the time difference--in functions needed to calculate thepositional uncertainty are stored in the corresponding memory cells inthe memory unit 7.

The measured Doppler shift may--but need not--be used to calculate theradial speed and thereby to verify the radial component of the targetspeed.

The required number of pulses may be calculated in different ways. Oneway is based on the known technique that combines the requirements ofdetection, range accuracy, range resolution and Doppler resolution.

The maximal time difference between the latest and the next measurementfor the current track is calculated while considering the fact that theposition uncertainty region of the target is to be completely includedin the sector formed by one monopulse. The position uncertainty-regiongrows according to the right term of the formulas a₁ and a₂ ; see alsoFIG. 3. The searched time point is calculated according to formulas a₁and a₂.

The minimal time difference between the latest and the next measurementfor the current track is calculated while considering the workload andillumination strategy. High workload is a reason to wait a certainminimum amount of time between two illuminations of targets of a certaintype. As regards illumination strategy, rules for restricted radarillumination on targets of certain types are intended.

An optimal time difference between the latest and the next measurementfor the current track is calculated among other things as a function ofminimal and maximal time difference. The optimal measurement timedifference may, for example, be chosen so that a certain percentage ofthe difference between the minimal and maximal time difference is lessthan the optimal time difference.

The scheduling unit 2 is formed by a plurality of calculation unitswhich may be connected according to FIG. 2.

A first calculation unit 2.1, selects the maximal PRF value from a givennumber of PRF values.

A second calculation unit 2.2, based on this maximal PRF value,calculates zero or one time interval between the time difference limits.Facts about the storage of information about time intervals arediscussed in connection with the memory unit 7.

A third calculation unit 2.3, by using the selected PRF value from thefirst calculation unit 2.1, calculates zero, one or more time intervalsduring which the range-unambiguity condition is satisfied. Thisrequirement means that the radial extension of the uncertainty-region ofthe position of the target is completely included in one singlerange-unambiguity interval during the time of the measurement. This maybe calculated in the following way, as further demonstrated in FIG. 3.

The time point for moving the target to the closest edge of theuncertainty-region is given by formula a₁ and the time point for movingthe target to the furthest edge of the uncertainty-region is given byformula a₂, where n=0, 1, 2, 3 . . . . The other variable names areexplained below. For an approaching target (i.e. a target thatapproaches the radar) the first time point, t in a₁, constitutes theupper limit for a time interval and the second time point, t in a₂,constitutes the lower limit for a time interval. For a departing targetthe situation is reversed. At most one time interval can be formed foreach value of n.

    n*c/(2*prf)+t.sub.pulse <r.sub.0 +t*v.sub.r μ.sub.l (t) (a.sub.1)

    r.sub.o +t*v.sub.r -μ.sub.u (t)<(n+1)* c/(2* prf)+t.sub.pulse <(a.sub.2)

where

n=an integer greater than or equal to zero,

c=the speed of light,

prf=pulse repetition frequency,

t_(pulse) =length of measurement time interval,

r₀ =range at the latest measurement,

t=time interval between the latest and next measurement,

V_(r) =the radial speed of the target, positive only if the target-radarrange increases,

μ_(l) (t)=the range between the straight-line flight path and theclosest edge of the uncertainty-region, which may be calculated by theKalman filter but also may be approximated to k_(l) *t², k_(l) >0.

μ_(u) (t)=the range between the straight-line flight path and thefurthest edge of the uncertainty-region, which may be calculated by theKalman filter but also may be approximated to k_(u) *t², k_(u) >0.

A fourth calculation unit 2.4, by using the selected PRF value from thefirst calculation unit 2.1 and information about the radial speed of thetarget, calculates whether Doppler blindness prevails; see formula (b).If this is the case with the selected PRF value, an empty time intervalsequence is produced, i.e. a sequence with zero time intervals.Otherwise a sequence with one time interval is produced, having thelimits zero and positive infinity, respectively.

The following formula for Doppler blindness indicates the blind freepart of each range rate unambiguity interval,

    m*λ*prf/2+d.sub.mar <v.sub.r <(m+1)*λ*prf/2-d.sub.mar(b)

where

m=integer greater than or equal to zero,

λ=carrier frequency,

prf=pulse repetition frequency,

d_(mar) =blind marginal in each end of the Doppler interval,

v_(r) =the radial speed of the target, positive only if the target radardistance increases.

A target which has a speed within a blind spot will be made invisible tothe radar. Since the range rate unambiguity interval depends on the PRFvalue, a target which was made invisible with one PRF value may becomevisible with another PRF value.

A fifth calculation unit 2.5, by using the selected PRF value from thefirst calculation unit 2.1, calculates zero, one or more time intervalsduring which cross-overs do not occur. These time intervals forms asequence of time intervals. Cross-overs are illustrated in FIG. 4.Cross-over is a state when the uncertainty region for one target ispartly or completely hidden by the uncertainty-region for anothertarget. The geometry for this state is time dependent. Simplifiedcalculations of cross-overs might be used. It might prevail at thosetime points when two or more targets have the same expected bearing,added with such time intervals on both sides of this time point whichdepend on the mutual relation between the directions of the targets. Ifthe targets head in nearly the same direction, the cross-over periodpersists for a longer time.

A sixth calculation unit 2.6, by using the selected PRF value from thefirst calculation unit 2.1, calculates zero, one or more time intervalswhich are not already allocated for update measurement of other targets.These time intervals form a sequence of time intervals.

It should be noted that information about time intervals for updatemeasurement of other targets may be obtained from track memory cells for"PRF" and "remaining time to measurement", as discussed in connectionwith memory unit 7, or be administered in a special sequence of timeintervals for occupied time intervals.

Zero, one or more seventh calculation units 2.7.j, where j=1,2,3, . . .,m, use the selected PRF value from the first calculation unit 2.1 tocalculate a sequence of time intervals during which measurement can notbe carried out for some other reason. One such reason is that a certainpart of each time unit must be allocated for search. The complementaryset (see definition below) for such allocated time intervals forms asequence of allowed time intervals for seventh calculation unit 2.7.j.Search is an activity, which consists of searching for new targets indefined areas. This comprises a method which, prior to scheduling trackupdates, allocates a specific percentage of each selected future timeinterval for search.

The complementary set to a sequence S of time intervals is a sequence Tof time intervals such that exactly those time points which are notincluded in any time interval in S are included in a time interval in T.

An eighth calculation unit 2.8 performs an intersection operation (seedefinition below) of all those time intervals or sequences of timeintervals which have been obtained from the second through seventhcalculation units 2.2-2.7. The result is a sequence of time intervals.

The intersection of two time interval sequences S₁ and S₂ is a timeinterval sequence T such that exactly those time points which areincluded both in a time interval in S₁ and a time interval in S₂ areincluded in a time interval in T. This might be writtenT=intersection(S₁,S₂). The intersection of several time intervalsequences S₁, S₂, . . . , S_(n), n>2, is then T_(n), which is achievedfrom a recursive process T_(n) =intersection(T_(n-1), S₂) for all n>2and where T₂ =T is obtained as described in the previous paragraph.

A ninth calculation unit 2.9 calculates the time for update measurementby using the sequence obtained from the eighth in unit 2.8.

First all intervals are cancelled whose length is smaller than themeasurement time length, which can be calculated from the followingformula

    T=2*f*w/prf                                                (c)

where

T=length of the measurement interval,

f=a positive integer whose value depends on the needed effect ascalculated by the radar equation,

w=integer which gives the number of pulses,

prf=pulse repetition frequency.

If at least one time interval then remains, calculation is made of thebeginning of a time interval whose length equals the measurement timelength and whose middle point lies as close as possible to the optimaltime, which is calculated in the track prediction unit 1 and is anoptimal time difference between minimum and maximum measurement timedifference. This value of the beginning of the measurement time intervalis stored in the memory space for "remaining time to next measurement"in the memory unit 7.

A tenth calculation unit 2.10, if no time interval according to theninth calculation unit 2.9 remains and there are more PRF values to try,deletes the last selected PRF value from the set of PRF values to try.

If, however, no time interval according to the ninth calculation unit2.9 remains and no more PRF values remain in the set of PRF values, thenresolution is instead performed by resolution unit 5.

The calculation units 2.2-2.7 are independent of each other. They may bearranged in parallel as shown in FIG. 2, or in sequence, in which caseintersection is performed at each step between the input time intervalsequence and the time interval sequence during the times of which thecondition is true. If these steps are performed sequentially, the needfor unit 2.8 disappears and the time interval sequence from unit 2.7 istransferred directly to the calculation unit 2.9.

An alternative to this, which takes a longer time to perform is to makean adjustment considering the following soft desirable requirements,namely,

minimization of the measurement time length,

closeness to the optimal time point and

centering of the Doppler interval whose limits are calculated withformula (b).

In order to be able to consider these desirable requirements and findthe PRF value which gives the best combination of desirablerequirements, each requirement may be weighted. This is done byintroducing three weight factors and then storing in one memory cell perPRF value--for all PRF values which give at least one possible timeinterval according to the ninth calculation unit 2.9--the value of thesum of three products. The products are achieved respectively bymultiplication of the inversed measurement length and the first weightfactor number 1, multiplication of the distance to the optimal timepoint and the second weight factor number 2, and multiplication of thedistance to the center of the Doppler interval and the third weightfactor number 3. After performing these calculations for all PRF values,that PRF value is selected which gives the smallest sum, together withthe corresponding measurement time interval. This consideration amongsoft desirable requirements may be selectable, i.e. an object for on/offswitching.

The assignment of values to weight factors may be static or dynamic. Ifstatic assignment is used, then the weight factors have constant values.If dynamic assignment is used, then the weight factors might be changedby an operator command or controlled by the transmitter/receiver byletting the weight factor for centering of the Doppler interval increaseas the signal-to-noise ratio decreases. A suitable initial valueassignment for the weight factors is 10⁻¹, 1 and 1 respectively.

The track selection unit 3 identifies--through comparing investigationamong all memory cells which for different tracks contain informationabout the remaining time to next measurement--the track i of all trackswhich has the smallest remaining time to next measurement K_(i),decreases the value for remaining time to next measurement for all othertracks by the same number K_(i) and transfers the number of the currenttrack, i, to the transmitter/receiver unit 4. The number K_(i) indicatesan absolute (future), time point namely the time point Now +K_(i), whereNow is the absolute time point for the latest measurement of the radar.

The transmitter/receiver unit 4 consists of transmitter, receiver andantenna for a phased array radar system. The transmitter calculatesdistance and angle according to functions which earlier have beendecided by the track prediction unit 1 for the current track. The numberof pulses required for measurement is calculated considering the energyrequired for target illumination. The PRF value which has beencalculated and stored in the state vector is used. The phase shift inthe different elements of the antenna is calculated so that a coherentpulse train is directed into the desired direction. Thetransmitter/receiver unit 4 stays in a delay state, and at the timepoint Now +K_(i) the transmission of the pulse train is started. Thereceiver infers the distance to the target by calculation of the timedelay of the received pulses. Further, distance and radial speed arecalculated. These data are registered at the state vector of the track.The value of the absolute time point for the latest measurement, Now, isupdated by addition of the value for K_(i).

If a target echo is not detected, a new attempt to take a measurement isimmediately made, which means that the values for minimal measurementtime difference and optimal time difference are set to zero, andscheduling unit 2 is started anew. Such renewed measurement attemptsafter a missed detection are normally repeated a few times (0-2). If atarget echo is still not detected, the track is deleted, which meansthat it is not studied any further by this or other units. In renewedmeasurement attempts alternative target track hypothesises may be used.

The resolution unit 5 calculates the resolution frequencies and possibletime intervals for resolution. Resolution means that the target isilluminated sequentially with a small number (s) of pulse trains withdifferent PRF values. At most m of these pulse trains (m<s) are allowedto have overlapping intervals for range blindness (that is the blindtime interval between two adjacent range-unambiguity intervals) in theuncertainty region of the target. The calculation in resolution unit 5is performed only if scheduling unit 2 could not deliver a measurementtime interval for a single PRF value.

The memory unit 7 consists of a memory having the required memory cells.These calls can be grouped according to the data stored within them.

A number of memory cells 7.1 may be designated for storing the values ofall selectable PRF values. These values are constant and used byscheduling unit 2.

A memory cell 7.2 may be designated for storing the absolute time pointNow for the latest measurement of the radar. The content in this cell isused and changed by the transmitter/receiver unit 4.

A number of groups of memory cells 7.3 may be designated, with, eachgroup representing a time interval. Each such group of cells holdsinformation about the start and end of the time interval, and a tracknumber and a pointer to another memory cell group which also representsa time interval--the pointer is a number or an address to the other cellgroup. A group of this type is a time interval object. A chain of timeinterval objects linked together represents a time interval sequence.Time interval objects are allocated and processed by the scheduling unit2. Time interval sequences are also created and maintained.

A group of memory cells 7.4 may be designated for storing the statevector for each track. This vector has elements for:

    ______________________________________                                        track number i  is assigned at track initiation by unit 6.                                                                           Each track has a                       unique number.                                                distance at the latest                                                                                         is calculated by unit 1; used by unit                        4.                                                            measurement                                                                   bearing at the latest                                                                               is calculated by unit 1; used by unit 4.                measurement                                                                   speed at the latest                                                                                   is calculated by unit 1; used by unit 4.              measurement                                                                   course at the latest                                                                                  is calculated by unit 1; used by unit 4.              measurement                                                                   growth factors k.sub.i, k.sub.u,                                                               is calculated by unit 1; used by unit 4.                     for positional uncertainty                                                    minimal (measurement) time                                                                      is calculated by unit 1; used by unit 2.                    difference may be constant or                                                 depend on the target                                                          volume;                                                                       maximal (measurement)                                                                            is calculated by unit 1; used by unit 2.                   time difference                                                               optimal time point                                                                                                 is calculated by unit 1; used by                         unit 2.                                                       function for calculation of how a                                                                   is calculated by unit 1; used by unit 4.                predicted position depends                                                    on the time difference between                                                the last and next measurement                                                 function for calculation of how                                                                     is calculated by unit 1; used by unit 2.                predicted positional uncertainty                                              depends on the measurement                                                    time difference                                                               radar target area                                                                                                is calculated by unit 4; used by unit                      4.                                                            planned PRF value for the                                                                                is calculated by unit 2; used by unit 4.           next measurement                                                              planned measurement time                                                                                  is calculated by unit 2: used by units            length for the                        2 and 3.                                next measurement                                                              remaining time to the next                                                                      the time difference between the time                        measurement K.sub.i                                                                                    point for start of next measurement                                                              of this trace and the time                        point                                                                                                     for the latest measurement                        which is                                                                                                  stored in the memory cell                         Now; K.sub.i                                                                                              is calculated by unit 2; used                     and                                                                                                       changed by unit 3.                time interval sequence N for                                                                  is calculated and used by units 2 and 3.                      possible measurement                                                          (see point 7.3);                                                              ______________________________________                                    

It should be noted that, with regard to this last element, there is oneimplementation version with and one without a special time intervalsequence for occupied time intervals. If this sequence is present, datafor the time intervals for the next measurement for each track will bestored in this element. If the special time interval sequence foroccupied time intervals is not present, time data for next measurementof each track might instead be stored only in the memory cell forremaining-time-to-next-measurement K_(i).

A group of memory cells 7.5 may be designated for storing those dataabout antenna and effect that are needed in the radar equation. Thisdata is used for calculation of the pulse number in thetransmitter/receiver unit 4.

The units 3, 4, 1 and 2 (in this order) constitute a sequence of unitswhich normally process one target track per turn. As a first exception,if the PRF value can not be calculated in unit 2, resolution isperformed in unit 5 as well. As a second exception, if no echo from thetarget is detected in the transmitter/receiver unit 4, a new schedulingis made by unit 2 without any intermediate processing in unit 1. If noecho is detected even after repeated measurement attempts, the targettrack is finished.

I claim:
 1. A phased array radar system for target tracking having atransmitter/receiver unit that forms a waveform and directs a beamtoward a target, comprising: a track initiation unit for initiating newtracks, the track initiation unit connected to a track prediction unitwhich predicts an expected position and a calculated positionuncertainty of the target as a function of time and a minimal, a maximaland an optimal time difference to a next measurement, a scheduling unitconnected to the track prediction unit which performs an independentcalculation of a sequence of possible time intervals to the nextmeasurement for each one of two conditions, namely that the measurementtime difference will be placed between the calculated minimal andmaximal time differences and that range-unambiguity will prevail, andthen performs an intersection operation between the so calculatedsequences of time intervals in order to calculate the optimal timedifference to the next measurement, a track selection unit connected tothe scheduling unit, which selects that track which has a shortestremaining time interval, K_(i), to the next measurement and decreases atime interval to the next measurement for all other tracks with K_(i),the track selection unit connected to the transmitter/receiver unitwhich, after directing the beam, registers an echo from the target andcalculates values for distance, speed, and bearing, and for uncertaintyin the distance and speed calculations, which values are transferred tothe track prediction unit.
 2. The radar system according to claim 1,further comprising a resolution unit connected to the scheduling unitfor calculating resolution frequencies and possible measurement timesfor resolution if the scheduling unit is not able to calculate any pulserepetition frequency.
 3. The radar system according to claim 1, furthercomprising a memory unit connected to the track prediction, scheduling,track selection, transmitter/receiver, and track initiation units whichstores needed facts for calculations and results of the calculations. 4.The radar system according to claim 3, wherein the scheduling unitcalculates the optimal time difference to the next measurement by,firstly selecting a maximal possible pulse repetition frequency (PRF)value among a number of predefined values, secondly calculating thatsequence of time intervals for each condition, which covers exactlythose time points during which the two conditions are satisfied, thirdlyperforming an intersection operation on the so calculated sequences oftime intervals which generates a time interval sequence N, fourthlycalculating a measurement time interval whose length equals ameasurement time length, is placed entirely within the time intervalsequence N and whose central point lies close to an optimal time point,and fifthly storing a starting time point for the measurement timeinterval in that cell of the memory unit which storesremaining-time-to-measurement, K_(i), in a state vector of track i. 5.The radar system according to claim 4, wherein, if no measurement timeinterval has been obtained and more PRF values remain, the schedulingunit eliminates a latest selected PRF value, whereafter calculation ofthe optimal time difference in the scheduling unit is repeated.
 6. Theradar system according to claim 5, wherein, if no PRF value remainsamong predefined PRF values, the scheduling unit signals the resolutionunit to perform resolution.
 7. The radar system according to claim 3,wherein the memory unit contains memory cells which store values for allselectable PRF values, data about an absolute time point for a latestmeasurement of the radar, data about time interval sequences, and dataabout a current state of every track including a memory address to atime interval sequence for possible measurement time points and memorycells for the minimal, the maximal and the optimal time differences tothe next measurement respectively and remaining-time-to-measurement,K_(i).
 8. The radar system according to claim 4, wherein the schedulingunit calculates an additional sequence of time intervals for a newcondition and performs an intersection operation on this additional timeinterval sequence and the sequences achieved from the two conditions,where the new condition relates to Doppler blindness avoidance.
 9. Theradar system according to claim 4, wherein the scheduling unitcalculates an additional sequence of time intervals for a new conditionand performs an intersection operation on this additional time intervalsequence and the sequences achieved from the two conditions, where thenew condition relates to cross-over avoidance.
 10. The radar systemaccording to claim 4, wherein the scheduling unit calculates anadditional sequence of time intervals for a new condition and performsan intersection operation on this additional time interval sequence andthe sequences achieved from the two conditions, where the new conditionrelates to freedom from overlapping.
 11. The radar system according toclaim 4, wherein the scheduling unit calculates an additional sequenceof time intervals for a new condition and performs an intersectionoperation on this additional time interval sequence and the sequencesachieved from the two conditions, where the new condition relates tosearch.
 12. The radar system according to claim 4, wherein thescheduling unit, for all those PRF values that give time intervalsequences with at least one time interval of sufficient length, firstly,calculates the length of the measurement time interval, minimum distanceto the optimal time point and distance to a Doppler central point,secondly, fuses those values by multiplying an inverted measurement timelength with a first weight factor, the minimum distance to the optimaltime point with a second weight factor and the distance to the Dopplerinterval central point with a third weight factor and summing, andthirdly, selects that PRF value which gives a smallest so calculatedsum.
 13. The radar system according to claim 12, wherein the schedulingunit uses weight factors of 10⁻¹, 1 and 1 for the first, second andthird weight factors, respectively.
 14. The radar system according toclaim 1, wherein the scheduling unit performs a new series ofcalculations with parameter values for minimal and optimal timedifference set to zero, if the transmitter/receiver unit (4) can notregister any echo from the target.
 15. The radar system according toclaim 14, wherein the track prediction unit changes a track hypothesisbetween each new measurement.